"Ground relay" circuits (ground short-circuit protection) and "differential relay" circuits (phase-to-phase short-circuit protection) are known in the prior art. However, such known circuits are complicated and therefore costly, whereas my invention provides equivalent protection with a simpler and less costly circuit.
In the prior protection systems known to me, short-circuit protection required a ground relay connected to a ground current transformer and three differential relays connected to three differential current transformers. The system of my invention reduces the number of relays and current transformers required to accomplish the desired result and therefore not only reduces cost but also minimizes maintenance problems.
Accordingly, my invention relates to a simplification in the control apparatus for short-circuit protection of an electric motor or an electric generator, and particularly to protection for a ground short-circuit and/or for a short-circuit between two or more phases in the motor.